A triangle that its legs and hypotenuse at an exactly 3:4:5 ratio has to be, by defintion, at 90 degrees or 'square'.

This means if you measure along the inner (house side)edge of your ledger board from its outer edge toward the inner part of the deck 3feet, then measure out from the ledger board along the outsidemost joist 4feet, then join these two points with an exactly 5foot long tape or peice of wood, the angle will be at exactly 90 degrees.